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Accumulator Pump.

low-pressure cylinder; and each engine works its neighbour's
valves. Thus the lever c of the opposite engine moves the rods
D and T, from which a system of link work connects to Corliss
valves, the fulcra above and below the cylinders being used re-
spectively for the exhaust valves F F, and steam valves E E. Aftei
use in both cylinders, the steam exhausts into the condenser G,
from which the water and vapour is withdrawn by the air pump H,
and delivered into the hot-well s. The pump itself needs no
description, but special attention must be drawn to the means
by which the driving force is so equalised as to be nearly uniform
when delivered to the plungers. Compensating cylinders L L, or
' pots/ rocking on pipe trunnions, contain water under a steady
pressure of about 200 Ibs. per sq. in., and have plungers pivoted to
the pump rod. This pressure constitutes a resistance to the steam
pressure during the first part, and an assistance during the second
part of the stroke, much in the same manner as the inertia of the
reciprocating parts, and the effect on the work diagram is shewn
at M. a and b are the indicator cards; c and d shew the pressure
exerted by the pot plungers, c assisting, and d opposing the steam
pressure : e is the combined effective-pressure diagrams from both
cylinders; and/is the resultant pressure on the pump-rod after
adding c and deducting d. The pot pressure is kept sensibly con-
stant by the intensifier N, whose larger piston p is under an air
pressure of about 75 Ibs. per sq. in. from the air vessel K, due to
the ^ater column ; and the smaller area Q is exerted on the water
I |                       in the pots. The arrangement constitutes a sort of governor, which

\ \ I                       controls the pump stroke, shortening it if a pipe happens to burst.

|                       To accurately adjust the pot pressure, some air is admitted under

|j                       p by cock R, causing a pressure of about 35 Ibs. per sq. in.   These

pumps are constructed by Messrs. Jas. Simpson & Co.

The Accumulator Pump, Fig. 722, is a double-acting pump,

requiring but one suction and one delivery valve. On account of
the great pressure to be resisted (750 Ibs. per sq. in.), an air
vessel is inadmissible. Referring to Fig. 663, in addition to Fig.
722, the piston A has twice the sectional area of rod B; so when
A. moves rightward, displacing the whole cylinder volume through
delivery valve D, half returns into B, and half goes to delivery J>ipe
b. A, returning leftward, draws a whole volume through suction